Object seen moving from west over Downtown Portland Oregon viewed from the corner of 1st Ave. and Hawthorne Street while walking to the shelter and just before getting on the Hawthorne Bridge. Object moved in a rapid zig zag motion and flashing white and red lights very rapidly and also duplicated colored lights before changing colors again. Red and White lights would alternately flash and then flash white a few times and then red a few times, it would then start alternating back to red and white again. It made a turn in an arc (half moon) to direct south. I lost contact for about one minute when it went behind a building but I viewed it again over the Willamette River when object soon disappeared. Object was approximately 1000-1500 feet in air and smaller than a dime in size. Displayed 2 colors and viewed for apporoximately 3 minutes. The object had no audible sound before its approach or after its passing. No other aircraft in viewing range.